The present invention relates to a portable visual display apparatus and, more particularly, to a head- or face-mounted display apparatus that can be retained on the observer's head or face.
A head-mounted display apparatus such as that shown in the plan view of FIG. 23 has heretofore been known (see U.S. Pat. No. 4,026,641). In the conventional head-mounted display apparatus, an image of an image display element 46, e.g., a CRT, is transferred to an object surface 12 by an image transfer device 25, and the image transferred to the object surface 12 is projected in the air by a toric reflector 10.
Another related art is disclosed in FIG. 9 of Japanese Patent Application No. 3-295874 (laid open on May 28, 1993 under publication No. 5-134208) by the present applicant. As shown in FIG. 24, the conventional display apparatus employs an ocular optical system 22 including a positive lens system for projecting an image of a two-dimensional image display element 21 in the air as an enlarged image, thereby projecting the image of the two-dimensional image display element 21 onto an observer's eyeball 23 as an enlarged image by the action of the positive lens system 22 only.
Incidentally, the conventional technique shown in FIG. 24 has an advantage in that the optical system needed therefor is simple in arrangement. However, it suffers from the problems that the amount to which the two-dimensional image display element 21 projects from the observer's head is disadvantageously large, and that it is not easy to switch over the outside world image and the observation image from one to the other, and further that if it is intended to ensure a wide field angle, the size of the positive lens system 22 increases, and thus the display apparatus increases in the overall size, making the observer feel uncomfortable when wearing it.
The problems to be solved by the present invention will be explained below more specifically.
For a head-mounted display apparatus, it is important to minimize the overall size thereof in order to make the observer feel comfortable when wearing it. Essential factors in determining the feeling given to the observer when wearing the display apparatus are the weight and center of gravity of the apparatus, and these factors are determined by the layout of the optical system. In the case of a direct-vision layout as shown in FIG. 24, the amount to which the apparatus projects from the observer's face is unfavorably large. Accordingly, the apparatus is heavy in weight, and the balance of weight is unfavorable. That is, the forward end of the observer's head is heavy, so that the observer cannot see forward unless he or she makes efforts to lift up his/her head at all times. Accordingly, the apparatus causes the observer to have a stiff neck.
To enable the observer to use the apparatus for a long time without fatigue, it is preferable to adopt an arrangement in which an ocular optical system including a reflecting surface, is disposed immediately in front of the observer's eyeball. With this arrangement, a two-dimensional image display element, an illuminating optical system, etc. can be disposed closer to the center of the observer's head. Accordingly, the center of gravity of the apparatus can be placed relatively close to the center of gravity of the observer's head.
Next, it is necessary to ensure a wide field angle in order to enhance the feeling of being at the actual spot which is given to the observer when viewing the displayed image. In particular, the stereoscopic effect of the image presented is determined by the field angle at which the image is presented (see The Journal of the Institute of Television Engineers of Japan Vol. 45, No. 12, pp. 1589-1596 (1991)). It is known that it is necessary in order to present a stereoscopic and powerful image to the observer to ensure a presentation field angle of 40.degree. (.+-.20.degree.) or more in the horizontal direction, and that the stereoscopic and other effects are saturated in the vicinity of 120.degree. (.+-.60.degree.). In other words, it is preferable to select a field angle which is not smaller than 40.degree. and which is as close to 120.degree. as possible. However, in a case where the above-described ocular optical system is a plane reflecting mirror, it is necessary to use an extraordinarily large two-dimensional image display element in order to make light rays incident on the observer's eyeball at a field angle of 40.degree. or more. After all, the apparatus increases in both the overall size and overall weight. The same problem arises in the arrangement wherein a concave mirror is used, and a two-dimensional image display element is disposed at the front focal point of the concave mirror, thereby projecting an image of the two-dimensional image display element in the air as an enlarged image by the concave mirror alone, as in the prior art shown in FIG. 23. That is, if a field angle of 40.degree. or more is provided in the above-described arrangement, rays of light emerging from the two-dimensional image display element strike on the observer's head. Therefore, it is impossible to realize the desired arrangement.